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arXiv 0802.0651cond-mat.stat-mech
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He II中涡环网络的动力学和超流Turbulence理论

Kinetics of a Network of Vortex Loops in He II and a Theory of Superfluid Turbulence

Sergey K. Nemirovskii

摘要:

基于涡环的合并和分裂动力学,发展了描述超流湍流的理论。由于非常频繁的重联,涡环(作为一个整体)的寿命不足以进行任何必要的演变,由于确定性的运动。相反,它们迅速合并和分裂,这些随机重组过程胜过其他较慢的动态过程。为了发展定量描述,我们采取涡环具有唯一自由度的布朗结构,该自由度是环的长度$l$。我们在Boltzmann型动力学方程的基础上对环数随长度$l$的分布函数$n(l)$进行了研究。利用碰撞积分中的特殊性质,我们求出了定常情况下动力学方程的类幂精确解。这种解不是(热力学上)平衡的,而是描述了涡环长度(或能量)在空间大小上有两个相互通量的状态。通量这个术语仅仅意味着由于重连而在不同大小的环之间重新分配长度(或能量)。通过分析这个解,我们得出了一些关于湍流超流氦中涡旋缠结的结构和动力学的结果。特别是,我们评估了平均曲率半径和重联事件的全速率。我们还研究了单位体积内涡环全长的演化--所谓的涡线密度。结果表明,这种演化服从著名的Vinen方程。从发展的方法的角度讨论了Vinen方程的性质。

英文摘要:

A theory is developed to describe the superfluid turbulence on the base of kinetics of the merging and splitting vortex loops. Because of very frequent reconnections the vortex loops (as a whole) do not live long enough to perform any essential evolution due to the deterministic motion. On the contrary, they rapidly merge and split, and these random recombination processes prevail over other slower dynamic processes. To develop quantitative description we take the vortex loops to have a Brownian structure with the only degree of freedom, which is the length $l$ of the loop. We perform investigation on the base of the Boltzmann type kinetic equation for the distribution function $n(l)$ of number of loops with length $l$. By use of the special ansatz in the collision integral we have found the exact power-like solution to kinetic equation in the stationary case. This solution is not (thermodynamically) equilibrium, but on the contrary, it describes the state with two mutual fluxes of the length (or energy) in space of sizes of the vortex loops. The term flux means just redistribution of length (or energy) among the loops of different sizes due to reconnections. Analyzing this solution we drew several results on the structure and dynamics of the vortex tangle in the turbulent superfluid helium. In particular, we evaluated the mean radius of the curvature and the full rate of the reconnection events. We also studied the evolution of the full length of vortex loops per unit volume-the so-called vortex line density. It is shown this evolution to obey the famous Vinen equation. The properties of the Vinen equation from the point of view of the developed approach had been discussed.

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